Micro/ultra-filtration membranes with a uniform pore diameter have several advantages when compared with conventional membranes with a wide distribution of pore diameters. Materials with a diameter larger than that of the pores will be held back with assurance, while a more uniform, quicker passage is assured for materials with a smaller pore size. The fixed pore size provides, for example, interesting applications in the biomedical and biotechnical engineering fields. In dialysis, the size of the opening for the passage of solutes (e.g., electrolytes, glucose, urea, creatine, barbiturates) must be adequate, while colloids and corpuscular constituents (protein, fats, blood cells, bacteria and viruses) are to be held back. To do this, the dividing limit must be approximately at the molecular weight 60,000. A significant improvement of modern dialysis membranes can be achieved with fixed pore diameters. Specific proteins can be separated in the pore range from 50 to 4,000 nm. Even larger pores with micron dimensions are in the order of magnitude of biological cells. Therefore, appropriate membranes could be used in cancer therapy. The pore range from 50 to 10,000 nm is also suitable for separating microorganisms (bacteria and viruses).
Biotechnology also offers many applications for micro/ultrafiltration membranes with a fixed pore size. In this case, it is important that certain biologically active substances be allowed to adhere to the surface of the membrane, while the conveying medium must be able to pass freely the products thus formed. Pore sizes similar to the ones described above may be used for the biomedical application described earlier.
Previous processes for the manufacture of membranes with a fixed pore size include the mechanical stamping of foils, ion bombardment in conjunction with chemical etching, and drilling with lasers.
Coining dies for mechanical engineering are made by means of surface relief technology. Here, photosensitive material (photoresist) is exposed to laser light in order to form the desired fine patterns on the surface. Following chemical development, these patterns are available as surface structures in the photoresist. The next step is to create a metallic copy of the photoresist structures by way of galvanoplastic technology. Such a process is described in U.S. Pat. No. 4,652,412.
In another technology, high-energy ion beams are shot through polymer foils. The ions leave channels in the foil that are subsequently etched out by chemical means. The holes, while uniform in diameter, are randomly distributed over the surface. Furthermore, superposition traces will have to be sorted out under the microscope in a later step. Reference is made to U.S. Pat. Nos. 3,303,085 and 3,612,871.
Finally, individual holes can also be burned directly into foils from different materials (metal, polymer, ceramics, etc.) by means of a laser. In most cases, infrared light of CO.sub.2 - or Nd-YAG lasers is used. The effect is dependent upon heat generation of the focused light, which vaporizes the material at the point of impact. However, due to thermal interaction, the remaining edges of the holes are damaged, since part of the material is melted open. This process did not produce hole diameters under 1 micron. In order to be able to perforate large foils and cover the whole area at high speed, the foil will have to be moved underneath the laser beam by a complex mechanism, if the laser heads are stationary and the beam has to be pulsed, correlated with an appropriate electronic system (e.g., U.S. Pat. No. 4,032,743).